Rectifying antennas (rectennas) for high power signals (.gtoreq.10 dBm) are used in satellite and radio relay systems. A rectifying antenna circuit achieves 80% to 90% RF to DC conversion efficiencies under these conditions. In contrast, rectifying antenna circuits for low power signals (.ltoreq.0 dBm), achieve much lower efficiencies. However, such low power signals are useful in passive RF transponder applications such as in RF identification (RFID) where the voltage required at the RF transponder is in the region of one volt and the current is on the order of tens of microamperes (.mu.A). Typically, RFID systems consist of a reader which sends an RF interrogation signal to a transponder, the transponder receiving the signal and transmitting a response signal containing the identification code of the transponder back to the reader so that the reader can identify the transponder. The RF energy received by a passive RF transponder is converted to DC power to drive the base band circuitry of the transponder to generate the response signal.
In conventional low power rectenna circuitry designs, to provide maximum power rectification, the impedance of zero bias Schottky diodes are matched to the receiving antenna. The matching circuit is achieved by intentionally selecting an antenna which has a reactance which resonates with the junction capacitance in the Schottky diodes or using inductance elements to match the impedance of the antenna with that of the Schottky diodes (see European Patent publication numbers EP-0 344 885 and EP-0 458 821). These methods of matching constrain the types of antennas and Schottky diodes used. Further, these approaches rely predominantly on the junction capacitance of the rectifying diodes within the voltage rectification circuit to achieve the voltage magnification. Since the antenna and diode are fixed, the resonant frequency cannot be tuned without redesigning the circuit or the antenna. Mis-matching--as a result of the tolerances inherent in the components in the printed circuit board of the transponder--results in frequency detuning which can cause an undesirable reduction in the optimised range of the passive RF transponder.
Another problem is that the capacitance of the diode which is dynamic in nature will be highly dependent upon the power level of the rectifying antenna circuitry and hence the current through the rectifying antenna circuitry. The resistance of the shunting Schottky diode is also dynamic being dependent on the current and will change the effective impedance of the Schottky diode depending upon the current level. These variations in the reactance of the Schottky diode can change the resonant frequency of the passive RF transponder and hence reduce available voltage magnification at a given frequency.
The present invention seeks to overcome the above problems by providing an improved voltage magnification circuit for passive RF transponders.
Accordingly, one aspect of the present invention provides a rectifying antenna circuit for a passive RF transponder comprising a series resonant circuit consisting of: an antenna; a voltage rectifier circuit including a diode; and a capacitance shunting the diode, the capacitance providing primarily a voltage amplification role and the diode providing primarily a rectification role.
In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings: